With the recent advance of digital technologies, a radiation image is converted into a digital image signal, and that digital image signal undergoes an image process, thus displaying the processed signal on, e.g., a CRT or printing it out. Upon photographing a radiation image, the X-ray dose is preferably as small as possible in consideration of the influences on a patient. But an image sensed with a small X-ray dose contains many quantization noise components, which disturb diagnosis.
For this reason, processes for removing such noise have been conventionally examined. For example, a noise removal process using a simple median filter, a method of removing noise by extracting high-frequency components using a smoothed image, and the like are done. In recent years, a multiplex frequency process for removing noise by segmenting an input image into a plurality of frequency bands, and executing independent processes for respective frequency bands has been examined.
In a filter process for removing noise by extracting high-frequency components using a smoothed image, since a single frequency band is used, noise removal cannot be effectively made if noise components are distributed to a broad frequency band. To avoid this, a plurality of filters having different sizes (i.e., different frequencies) may be simultaneously used. However, computation cost required for the process increases considerably. In order to maintain optimal frequency characteristics of a filter for noise removal, filter size adjustment corresponding to an object is indispensable, resulting in poor versatility.
The above problems can be greatly reduced using the multiplex frequency process represented by, e.g., discrete wavelet transformation in noise removal. However, since the multiplex frequency process executes uniform frequency processes in all spatial directions, for example, it cannot distinguish an edge having a direction component from isolated point noise having no direction component, and it is difficult for that process to remove noise while preserving the edge.
The present invention has been made in consideration of the aforementioned problems, and has as its object to effectively execute noise removal.